Electrokinetic pumping

ABSTRACT

The invention provides electrode-based pumps and methods of operating such pumps. In one embodiment, the invention provides an electrode-based pump situated in a fluid channel comprising a first electrode and a second electrode, wherein the first and second electrodes have a diameter from about 25 microns to about 100 microns and are spaced from about 100 microns to about 2,500 microns apart. In another embodiment, the invention provides an electrode-based pump comprising a first electrode, second electrode and third electrode.

This application is a continuation-in-part of U.S. application Ser. No.08/469,238, titled "Apparatus and Methods for Controlling Fluid Flow inMicrochannels," filed Jun. 6, 1995, now U.S. Pat. No. 5,632,876 and acontinuation-in-part of U.S. application Ser. No. 08/483,331, titled"Method and System for Inhibiting Cross-Contamination in Fluids ofCombinatorial Chemistry Device," filed Jun. 7, 1995, now U.S. Pat. No.5,603,351.

This application relates to electrode-based pumps, methods of operatingsuch pumps, and calibration devices for such pumps.

Recently, a number of academic articles have focused on the problemsassociated with conducting chemical reactions on a micro-scale. Thisliterature has discussed the possibility of managing such reactions onwafer-sized solid supports that have been etched to createmicrochannels. Reactor systems of this scale could allow multiplediagnostic or drug screening assays to be conducted in a transportabledevice that uses small amounts of reagents, thus reducing supply anddisposal costs.

One mechanism for developing new drugs not provided for by nature hasbeen dubbed "rational" drug design. This process looks at the structuresof biological macromolecules as determined by crystallography and at thestructures of pharmacological agents known to interact with thesemacromolecules. With the use of computer workstations, it was hoped thatnew pharmacological agents could be designed that had appropriatelypositioned functionalities for strongly interacting with themacromolecule. One difficulty with this approach is that growingcrystals appropriate for crystallographic structural determinations is atedious, empirical science. In many cases, it is unclear if appropriatecrystals can be grown (for instance, for the glycoprotein hormones sucha chorionic gonadotropin or other glycoproteins). Another difficulty isthat chemistry does not provide the malleable construction tools evokedby the phrase "design"; instead, chemical building blocks provide only alimited number of bond angles and lengths. For example, the structuralroutes by which a chlorine group might be positioned in particular partof a drug-binding pocket in the macromolecule may be many, while theadvantages or disadvantages of the ancillary structures needed toposition this group are hard to "rationally" evaluate.

Combinatorial chemistry seeks to create its own "evolutionary" processthat selects for compounds with the desired pharmacological activity.The key to making the process evolutionary is to generate large familiesof "mutants", in this case families of compounds with some chemicalrelatedness but with clear differences. The concepts of rational designmay be taken advantage of in selecting the families of compounds to beexplored by the combinatorial method.

Combinatorial chemistry seeks to generate new leads to classes ofcompounds that have potential pharmacological activity. Traditionally,such leads have been found by screening various plant or animal extractsfor pharmacological activity. Such extracts are tedious to obtain, mayhave very small concentrations of potentially useful compounds, and atbest only contain compounds selected by evolutionary pressures that mayhave nothing to do with the disease that is sought to be treated. Afteran extract has been identified, the process provides little informationas to the identity of the active ingredient.

Combinatorial chemistry seeks to create the large family of compounds bypermutation of a relatively limited set of building block chemicals.Preferably, the combinatorial method will create identifiable poolscontaining one or more synthetic compounds. These pools need not beidentifiable by the chemical structure of the component compounds, butshould be identifiable by the chemical protocol that created thecompounds. These pools are then screened in an assay that is believed tocorrelate with a pharmacological activity. Those pools that producepromising results are examined further to identify the componentcompounds and to identify which of the component compounds areresponsible for the results.

The follow-up protocol used to identify the active compounds in acombinatorial pool can also involve a combinatorial method. Forinstance, the promising pool could result from the reaction, first, of amixture of compounds A, B and C, which compounds do not react with oneanother, with compounds D, E and F, which compounds do not react withone another but do react with compounds A, B or C. Second, the resultingcompounds are reacted with compounds G, H and I. To narrow the possibleidentity of the active compounds in the pool, the A-D, A-E, A-F, B-D,B-E, B-F, C-D, C-E and C-F products can be separately created bycombinatorial chemistry and separately reacted with a the mixture of G,H and I. After this step, the sub-pool that is active in the screeningassay generally will contain a more limited family of compounds.

Once promising molecules are identified by combinatorial chemistry, theidentified molecules provide information that aides in the design offurther combinatorial experiments. The full array of promising compoundsidentified by combinatorial chemistry can provide valuable informationto guide traditional pharmaceutical chemistry efforts.

A popular tool in the emerging field of combinatorial chemistry is toattach the first chemical building blocks to a solid support, typicallya glass or polymeric support, such as the supports used in the wellknown Merrifield method for synthesizing polypeptides. This attachmentprovides a mechanism for quickly isolating product by simply washingaway reactants and related impurities and decoupling the product fromthe support. In some cases, the support-coupled product can be assayedfor pharmacological activity.

Miniaturization is usefully employed in combinatorial chemistry since:(i) workers generally seek compounds that are pharmacologically activein small concentrations; (ii) in creating a vast "evolutionary"assortment of candidate molecules it is preferable to have the numerousreactions well documented and preferably under the direction of alimited number of workers to establish reproducibility of technique;(iii) it is expensive to create a vast, traditionally-scaled syntheticchemistry complex for creating a sufficiently varied family of candidatecompounds; and (iv) substantial concerns are raised by the prospect ofconducting assays of the products of combinatorial chemistry at morestandard reaction scales. Miniaturization allows for the economic use ofrobotic control, thereby furthering reproducibility.

The wafer-sized devices described above can be ideal for combinatorialchemistry, allowing for numerous synthetic chemistry reactions to beconducted substantially under computer control using only smallquantities of reagents. However, the academic literature advocating suchmicro-scale devices has not adequately addressed fundamental issues inconducting combinatorial chemistry at this scale: for instance, how doesone manage to effectively pump fluids in such a device to the multitudeof microscaled reaction cells (e.g., 100 to 100,000) in the device? Thepresent invention provides a pump that can be incorporated within suchdevices.

SUMMARY OF THE INVENTION

The invention provides electrode-based pumps and methods and devices foroperating such pumps. In one embodiment, the invention provides anelectrode-based pump situated in a fluid channel comprising a firstelectrode and a second electrode, wherein the first and secondelectrodes have a diameter from about 25 microns to about 100 micronsand are spaced from about 100 microns to about 2,500 microns apart. Inanother embodiment, the invention provides an electrode-based pumpcomprising a first electrode, second electrode and third electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a voltage pulse pattern used to power an electrode-basedpump useful in the liquid distribution system of the invention.

FIGS. 2A and 2B show the field strength and orientation at numerouspoints about electrode-based pumps.

FIG. 3 shows a calibration device.

FIG. 4 shows another calibration device.

FIG. 5 displays a cut-away view of a liquid distribution system that canbe used with the invention.

FIG. 6 displays a distribution plate of the liquid distribution systemof FIG. 5.

FIG. 7 displays an expanded view of a portion of the distribution plateof FIG. 6.

FIG. 8 shows a capillary barrier between a first distribution channeland a buffer channel.

FIGS. 9A-9D show various capillary barrier designs.

FIG. 10 shows a device for conducting field assisted bonding of plates.

FIG. 11 shows a digital driver for powering the electrode-based pumps.

FIGS. 12A and 12B show a channel device having electrode-based pumps.

FIG. 13 shows a liquid distribution system design pursuant hydrologicliquid distribution system.

FIG. 14 shows a reaction cell having a heater and a thermocouple.

FIGS. 15A and 15B show a valve design.

DEFINITIONS

The following terms shall have the meaning set forth below:

addressable

a reaction cell or channel is "addressable" by a reservoir or anotherchannel if liquid from the reservoir or other channel can be directed tothe reaction cell or channel.

adjacent

"adjacent" as used in these situations: (i) a first structure in one ofthe plates is adjacent to a second structure in the same or anotherplate if the vertical projection of the first structure onto the plateof the second structure superimposes the first structure on the secondor places it within about 250 μm of the second; and (ii) groupings oftwo or more channels are adjacent to one another if each channel is insubstantially the same horizontal plane, and all but the outside twochannels in the grouping are adjacent (in the sense defined in (i)above) to two neighbor channels in the grouping. Preferably, under item(i), a first structure is adjacent to a second structure if the verticalprojection of the first structure onto the plate of the second structuresuperimposes the first structure on the second or places it within about150 μm of the second.

capillary dimensions

dimensions that favor capillary flow of a liquid. Typically, channels ofcapillary dimensions are no wider than about 1.5 mm. Preferably channelsare no wider than about 500 μm, yet more preferably no wider than about250 μm, still more preferably no wider than about 150 μm.

capillary barrier

a barrier to fluid flow in a channel comprising an opening of thechannel into a larger space designed to favor the formation, by liquidin the channel, of an energy minimizing liquid surface such as ameniscus at the opening. Preferably, capillary barriers include a damthat raises the vertical height of the channel immediately before theopening into the larger space.

connected

the channels, reservoirs and reaction cells of the invention are"connected" if there is a route allowing fluid between them, which routedoes not involve using a reaction cell as part of the link.

directly connected

reservoirs and horizontal channels are "directly connected" if they areconnected and either (1) no other channel is interposed between them or(2) only a single vertical channel is interposed between them.

flow preference

the direction that a liquid pumps under the influence of anelectrode-based pump having two symmetrically situated rod-shapedelectrodes.

hole diameter

because techniques for fabricating small holes often create holes thatare wider at one end than the other (for instance, about 50 micronswider), the hole diameter values recited to herein refer to thenarrowest diameter.

horizontal, vertical, EW, NS

indications of the orientation of a part of the distribution systemrefer to the orientation when the device is in use. The notations "EWaxis" and "NS axis" are in reference to FIGS. 1, 2, 3 and 7, where an EWaxis goes from right to left and is perpendicular to the long axis ofthe page and a NS axis is from top to bottom parallel to the long axisof the page.

independent

channels, reservoirs or reaction cells that are not connected.

offset

two sets of channels are "offset" when none of the channels in the firstsuch set is adjacent to any of the channels in the second set.

perpendicular

channels in the distribution plate are perpendicular even if primarilylocated on separate horizontal planes if their vertical projections ontothe same horizontal plane are perpendicular.

reservoir

unless a different meaning is apparent from the context, the terms"reservoir" and "fluid reservoir" include the horizontal extensionchannels (sometimes simply termed "extensions") directly connected tothe reservoir or fluid reservoir.

second reservoir extension channels

these extension channels include the distribution channels that maybranch off of these extension channels.

substantially the length of one of the horizontal dimensions

at least about 70% of one of the major horizontal dimensions (e.g. theEW or NS dimensions illustrated in the Figures) of the distributionplate.

U-plumbing channel

a channel designed to connect at least two channels or reservoirs suchthat the liquid level in one of the connected channels or reservoirswill equalize with the liquid level in the other connected channel orreservoirs due to hydrological forces. U-plumbing channels typicallyhave vertical channels that connect channels or reservoirs located in ahigher vertical plane with a substantially horizontal channel segment ofthe U-plumbing channel located in a lower plane--these vertical andhorizontal segments together comprise the U-plumbing channel. The feederchannels of the invention are typically U-plumbing channels.

DETAILED DESCRIPTION

A. Electrode-based Pumps

At least two types of such electrode-based pumping has been described,typically under the names "electrohydrodynamic pumping" (EHD) and"electroosmosis" (EO). EHD pumping has been described by Bart et al.,"Microfabricated Electrohydrodynamic Pumps," Sensors and Actuators,A21-A23: 193-197, 1990 and Richter et al., "A MicromachinedElectrohydrodynamic Pump," Sensors and Actuators, A29: 159-168, 1991. EOpumps have been described by Dasgupta et al., "Electroosmosis: AReliable Fluid Propulsion System for Flow Injection Analysis," Anal.Chem., 66: 1792-1798, 1994. In the present application, pumping effectedwith electrodes is termed "electrokinetic pumping."

EO pumping is believed to take advantage of the principle that thesurfaces of many solids, including quartz, glass and the like, becomecharged, negatively or positively, in the presence of ionic materials,such as salts, acids or bases. The charged surfaces will attractoppositely charged counter ions in solutions of suitable conductivity.The application of a voltage to such a solution results in a migrationof the counter ions to the oppositely charged electrode, and moves thebulk of the fluid as well. The volume flow rate is proportional to thecurrent, and the volume flow generated in the fluid is also proportionalto the applied voltage. Typically, in channels of capillary dimensions,the electrodes effecting flow can be spaced further apart than in EHDpumping, since the electrodes are only involved in applying force, andnot, as in EHD, in creating charges on which the force will act. EOpumping is generally perceived as a method appropriate for pumpingconductive solutions.

EHD pumps have typically been viewed as suitable for moving fluids ofextremely low conductivity,. e.g., 10⁻¹⁴ to 10⁻⁹ S/cm. It has now beendemonstrated herein that a broad range of solvents and solutions can bepumped using appropriate solutes than facilitate pumping, usingappropriate electrode spacings and geometries, or using appropriatepulsed or d.c. voltages to power the electrodes, as described furtherbelow.

The electrodes of first pumps 360 and second pumps 361 used in theliquid distribution system described below preferably have a diameterfrom about 25 microns to about 100 microns, more preferably from about50 microns to about 75 microns. Preferably, the electrodes protrude fromthe top of a channel to a depth of from about 5% to about 95% of thedepth of the channel, more preferably from about 25% to about 50% of thedepth of the channel. Usually, as a result the electrodes, defined asthe elements that interact with fluid, are from about 5 microns to about95 microns in length, preferably from about 25 microns about to 50microns. Preferably, a pump includes an alpha electrode 364 (such asfirst electrode 360A or third electrode 361A) and a beta electrode 365(such as third electrode 360B and fourth electrode 361B) that arepreferably spaced from about 100 microns to about 2,500 microns apart,more preferably, from about 150 microns to about 1000 microns apart, yetmore preferably from about 250 microns to about 1000 microns apart, or,in an alternate embodiment, from about 150 microns to about 250 micronsapart. The separation of electrodes shall be measured from the centerpoints of the electrodes as they first protrude into their associatedfluid channel. In a particularly preferred embodiment, a gamma electrode366 (not shown) is spaced from about 200 microns to about 5,000 microns,more preferably from about 500 microns to about 1,500 microns, yet morepreferably about 1,000 microns from the farther of the alpha electrode364 and the beta electrode 365. In an alternative preferred embodiment,the pump has two additional electrodes comprising a gamma electrode 366(not shown) and a delta electrode 367 that are spaced from about 200microns to about 5,000 microns, more preferably from about 500 micronsto about 1,500 microns, yet more preferably about 1,000 microns apart.Where the electrodes are located in fluid channels that have bends, thedistances are measured along a line that defines the center line of thefluid channel. In contexts where relatively low conductivity fluids arepumped, voltages are applied across the alpha electrode 364 and the betaelectrode 365, while in contexts where relatively more highly conductivefluids are pumped the voltage is induced between gamma electrode 366 andone of alpha electrode 364, beta electrode 365 or delta electrode 367.The latter circumstance typically applies for solvents traditionallypumped with EQ pumping, although this invention is not limited to anytheory that has developed around the concepts of EHD or EQ pumping. Nofirm rules dictate which electrode combination is appropriate for agiven solvent or solution; instead an appropriate combination can bedetermined empirically in light of the disclosures herein.

The voltages used across alpha and beta electrodes 364 and 365 when thepump is operated in d.c. mode are typically from about 40 V to about2,000 V, preferably from about 50 to about 1,500V, more preferably ablyfrom about 100 V to about 750 V, yet more preferably from about 200 V toabout 300 V. The voltages used across gamma electrode 366 and alpha,beta or delta electrodes 364, 365 or 367 when the pump is operated ind.c. mode are typically from about 40 V to about 2,000 V, preferablyfrom about 50 to about 1,500V, more preferably ably from about 100 V toabout 750 V, yet more preferably from about 200 V to about 300 V. Thevoltages used across alpha and beta electrodes 364 and 365 when the pumpis operated in pulsed mode can be as indicated above for d.c. mode, butare typically from about 50 V to about 1000 V, preferably from about 100V and about 400 V, more preferably from about 200 V to about 300 V. Thevoltages used across gamma electrode 366 and the alpha, beta or gammaelectrode 364, 365 or 367 when the pump is operated in pulsed mode canbe as indicated above for d.c. mode, but are typically from about 50 Vto about 1,000 V, preferably from about 100 V and about 400 V, morepreferably from about 200 V to about 300 V. Preferably, the ratio ofpumping to current will be such that no more than about one electronflows into the solution adjacent to a first pump 360 or second pump 361for every 1,000 molecules that move past the pump 360 or 361, morepreferably for every 10,000 molecules that move past the pump 360 or361, yet more preferably for every 100,000 molecules that move past thepump 360 or 361.

It is believed that an electrode-based internal pumping system can bestbe integrated into the liquid distribution system of the invention withflow-rate control at multiple pump sites and with relatively lesscomplex electronics if the pumps are operated by applying pulsedvoltages across the electrodes. FIG. 1 shows an example of a pulseprotocol where the pulse-width of the voltage is τ₁ and the pulseinterval is τ₂. Typically, τ₁ is between about 1 μs and about 1 ms,preferably between about 0.1 ms and about 1 ms. Typically, τ₂ is betweenabout 0.1 μs and about 10 ms, preferably between about 1 ms and about 10ms. A pulsed voltage protocol is believed to confer other advantagesincluding ease of integration into high density electronics (allowingfor hundreds of thousands of pumps to be embedded on a wafer-sizeddevice), reductions in the amount of electrolysis that occurs at theelectrodes, reductions in thermal convection near the electrodes, andthe ability to use simpler drivers. The pulse protocol can also usepulse wave geometries that are more complex than the block patternillustrated in FIG. 1.

Another, procedure that can be applied is to use a number of electrodes,typically evenly spaced, and to use a travelling wave protocol thatinduces a voltage at each pair of adjacent electrodes in a timed mannerthat first begins to apply voltage to the first and second electrodes,then to the second and third electrodes, and so on. Such methods aredescribed in Fuhr et al., J. Microelectrical Systems, 1: 141-145, 1992.It is believed that travelling wave protocols can induce temperaturegradients and corresponding conductivity gradients that facilitateelectric field-induced fluid flow. Such temperature gradients can alsobe induced by positioning electrical heaters in association with theelectrode-based first pumps 360 and second pumps 361.

While not wishing to be restricted to theory, several theoreticalconcepts are believed to play a role in the mechanics of EHD pumping.The forces acting on a dielectric fluid are believed to be described by:##EQU1## where F is the force density, q is the charge density, E is theapplied field, P is the polarization vector, ε is the permittivity and ρis the mass density. Of the terms in the equation, the first and thirdare believed to be the most significant in the context of EHD pumping offluids. The first term (qE) relates to the Coulomb interaction with aspace-charge region. The third term (1/2E² ∇ε) relates to the dielectricforce which is proportional to the gradient in permittivity.

In low fields, i.e., the Ohmic region where current is linearlyproportional to voltage, the primary source of charges that will beacted upon by the electric field are believed to be primarily due toions from additives, ions from impurities and ions formed byautodissociation of molecules in the fluid. In intermediate fields, i.e.from beyond the Ohmic region to about 2 V/μm, the charges are believedto be primarily formed by dissociation and electrolylytic processes inthe fluid. In higher fields, the charges are believed to be determinedby injection processes at the electrodes, which electrodes injecthomocharges.

For the purposes of this application, positive (+) flow shall be flow inthe direction of the negative electrode, and negative (-) flow shall beflow in the direction of the positive electrode.

In a preferred embodiment of the invention, the controller 10 has adevice for storing data and stores the values of voltage and polaritysuitable for pumping a number of solvents.

Experimental results indicate that the properties of fluid flow (likedirection of flow) correlate well with the solvent's ability tostabilize and solvate the charged species injected or induced from theelectrodes. The direction of flow is believed to be determined by thepreference of the solvent to solvate either cations or anions. Thissolvation preference is believed to imply a greater shell of solventmolecules that will be dragged in an electric field, creating fluidmovement, when a field is applied to the electrodes of a first pump 360or a second pump 361. For example, a preferred solvation of cationscorrelates with a preference for fluid flow from the anode to thecathode (i.e., the positive direction). The degree of such a salvationpreference for a solvent is believed to depend on the ability of themolecules within the solvent to accept or donate hydrogen bonds. In oneaspect of the invention, for liquids whose pumping behavior has not yetbeen characterized, the controller will store initial pumping parametersestimated using on the Linear Solvation Energy relationships establishedby R. W. Taft and co-workers. See, Kamlet et al., J. Org. Chem., 48:2877-2887, 1983 and Kamlet et al., Prog. Phys. Org. Chem., 13: 485,1981. These workers have categorized solvents in terms of the followingparameters: U, the ability of the solvent to stabilize a stabilize acharge or dipole by virtue of its dielectic properties; α, the hydrogenbond donating ability of the solvent; and β, the hydrogen bond acceptingability of the solvent. These parameters are more fully defined in theabove-cited Kamlet et al. publications, from which these definitions areincorporated herein by reference.

Using a one mm capillary of circular cross-section, a pair of 50 micronrod-shaped, platinum electrodes perpendicularly inserted to a depth of500 microns into the capillary with a 500 micron separation powered by a400 V field, the direction of flow was determined for several solvents.The direction of flow and the α, β, π, ε and dipole moment values are asfollows:

    ______________________________________                                              dipole    Solvent direction α                             β                                   η                                        ε                                              moment    ______________________________________    ethanol -         0.83   0.77  .54  24.55 1.69    tetrahydro-            +         0      0.55  .58  7.58  1.75    furan    chloroform            -         0.44   0     .58  4.806 1.01    acetone +         0.08   0.48  .71  20.7  2.69    methanol            -         0.93   0.62  .6   32.7  2.87    2-propanol            +/-       0.76   0.95  .48  19.92 1.66    acetonitrile            +         0.19   0.31  .75  37.5  3.92    N-methyl-            +         0      0.77  .92  32.0  4.09    pyrrolidone    diethyl ether            +         0      0.47  0.27 4.335 1.15    1,2 dichloro            -         0      0     0.81 10.36 1.2    ethane    DMF     +         0      0.69  .88  36.71 3.86    ______________________________________

It is believed that the α and β values reflect the ability of thesolvent under an electric field to solvate a negative or positivecharged species, with the magnitude of α-β correlating with (-) flow,and the magnitude of β-α a correlating with (+) flow. According to oneaspect of the invention, the preferred direction of flow of a liquid canbe reversed from that predicted as above if the fluid has a differencein α and β values that is small but not zero and the electrode pair usedcreates an asymmetric field, such that the acting force on eitherpositive or negative charged species is enhanced. One such electrodepair has an alpha electrode 364 with that points in the direction ofintended flow and a beta electrode 365 that lines the walls of thechannel in which it is located. Preferably, the alpha electrode 364sufficiently points in the direction of flow such that its point definesa line that intersects the plane defined by the beta electrode 365.Preferably, the alpha electrode 364 ends in a point or wedge shape. Suchan electrode-based pump, fabricated in a 1 mm capillary, has been shownto be effective to pump 2-propanol in the direction pointed to by thealpha electrode 364 either when the voltage applied to the electrodesimplied a (-) direction of flow or, with somewhat weaker flow, when thevoltage applied to the electrodes implied a (+) direction of flow.

The asymmetric, electrode-based pump effective to pump 2-propanol in thedirection pointed to by the alpha electrode 364 is illustrated in FIG.2A. Alpha electrode 364 points from left to right in the figure. Betaelectrode 365 is a ring electrode that is flush with the sides of thecapillary 501. Using the QuickField program available from TeraAnalysis, Granada Hills, Calif., the electric field strengths andorientations at various points about the electrodes is indicated by thesize and orientation of the arrow at that point. It can be seen fromFIG. 2A that where alpha electrode 364 acts as a cathode, the area wherea solvated negative ion would be influenced by a strong field pushing itin the (+) direction is greater than the area where a strong field wouldpush a solvated positive ion in the (-) direction. The integration ofthese forces acting on these solvation spheres explains why asymmetricfields can be used to pump a liquid against its ordinary preferreddirection of flow. Thus, depending on the liquids to be pumped,asymmetric fields can be used to assure a given direction of flow.

For comparison purposes, FIG. 2B shows the field strength andorientation of a number of points about a more symmetrical pump havingalpha electrode 364 and beta electrode 365.

The pumping parameters of a liquid can be calibrated using a plug of theliquid disposed in a capillary that has an electrode-based pump and isangled uphill. If optical devices are associated with the capillary formonitoring the position of the plug, the velocity of pumped flow uphilland the velocity of gravity driven downhill motion can be measured. Withthese velocities and the angle of the capillary, the pressure applied tothe liquid can be calculated. (Fluid resistance, R=(8·μ·l)/πr⁴, where udefines viscosity and l=the length of the fluid plug; Pressure,P=RA(v_(up) -v_(down)), where A =cross-sectional area). The efficiencyof the pump can also be calculated (η=(q·ρ·Q·N_(A))/m·l, where q=chargeof e⁻, ρ=density of liquid, Q=flow rate=v_(up) ·A, m=mass of liquid, andI=current). The velocities can be measured with multiple single pointobservations of the location of either the front or rear interfaces ofthe plug using fixed LEDs and optical detectors or in a continuous modeusing a light and a silicon photodiode position sensor, such as a SL15or SC10 position sensor available from UDT Sensors, Inc., Hawthorne,Calif. With the latter method, the correlation between the signalproduced at the difference amplifier connected to the position sensormust be calibrated prior to experimental use.

FIG. 3 shows a calibration device having first optical detector 401,second optical detector 402 and third optical detector 403. These arepreferably photodiodes such as the OTS-254 phototransister availablefrom Opto Technnology, Inc., Wheeling, Ill. Not shown are correspondingfirst light source 411, second light source 412 and third light source413. These are preferably LEDs such as the Super Bright LEDs availablefrom Radio Shack. A capillary 420 is situated in mount 430. Mount 430can be manipulated to orient capillary 420 at an angle offset from ahorizontal orientation. First lead 441 and second lead 442 relay voltageto the electrode based pump 460 (obscured by mount 430) from thevoltage-generating portion 470 of controller 10. Electrical signalscorrelating with light detection are relayed to the signal processingportion 480 of controller 10 by way of first data lead 481, second datalead 482 and third data lead 483. In operation, the signals from firstoptical detector 401, second optical detector 402 and third opticaldetector 403 will show a transition point when the interface of a plug11 of liquid in capillary 420 goes by the respective detector (401, 402or 403). The timing of these transitions provides a measure of thevelocity of movement of the plug 11.

FIG. 4 shows a calibration device having a position sensor 490 whichgenerates electrical signals based on the distribution of light fromlight source 491 that impacts the surface of the position sensor 490.Light source 491 has a power source 492. Leads 440 that relay voltage topump 460 are shown schematically. The voltage is controlled by thecontroller 10 through its pulse generator 471 and voltage driver 472.Electrical output from the position sensor is relayed via leads 484 tothe data acquisition module 485 of controller 10. It will be recognizedthat the signal from the position sensor can be calibrated so that itcan be processed to determine the position of the interface of plug 11.This information can be used to calculated the velocity of the movementof plug 11.

The pumping parameters for a number of solvents have been determined inthe 1 mm capillary described above, asfollows:______________________________________ Flow rate, Q Pressure, Pelectrical efficiency,Solvent μl/sec N/m² η,molecules/e______________________________________acetone 14.56 16.33 1.9× 10⁶methanol 24.46 26.32 9.7 × 10⁴1-propanol 16.39 74.89 4.2 ×10⁵diethyl ether 18.44 20.45 5.8 × 10⁸1,2 dichloroethane 14.24 46.55 2.9× 10⁷______________________________________

Another aspect of pumping is the observation that fluids that areresistant to pumping at a reasonable field strength can be made moresusceptible to electrode-based pumping by adding a suitable pumpingadditive. Preferably, the pumping additive is miscible with theresistant fluid and can be pumped at high pressure, P, high flow rate,Q, and good electrical efficiency, η (i.e., molecules pumped perelectron of current). Generally, the pumping additive comprises fromabout 0.05% w/w to about 10% w/w of the resulting mixture, preferablyfrom about 0.1% w/w to about 5% w/w, more preferably from about 0.1% w/wto about 1% w/w. Carbon tetrachloride and cyclohexane do not pump usingthe electrode pump situated in a capillary described above at a voltageof 2,000 V. By adding 0.5% w/w acetone or methanol as a pumpingadditive, both of these fluids can be pumped at a voltage of 1,000V. Insome cases, it is desirable to reverse the preferred flow direction of aliquid by mixing with it a pumping additive that strongly pumps in thedesired direction. In all cases, pumping additives are selected on thebasis of their pumping characteristics and their compatibility with thechemistries or other processes sought to be achieved in the liquiddistribution system.

The electrode-based pumps of the invention can be operated to as a valveto resist flow in a certain direction by operating the pumps to counterthe unwanted flow. To power the electrode-based pumps, one or moredigital drivers, consisting of, for example, a shift register, latch,gate and switching device, such as a DMOS transistor, permits simplifiedelectronics so that fluid flow in each of the channels can be controlledindependently. Preferably, each digital driver is connected to multipleswitching devices that each can be used to control the pumping rate of aseparate electrode-based pump.

The invention includes employing an electrode-based pump to move reagentselected from the group consisting solutions of amino acids, protectedamino acids, nucleotides, protected nucleotides, carbodiimides, reactivederivatives of N-protected amino acids and phosphoamidite derivatives ofnucleotides. The carbodiimides are preferably C2 to C12 arylcarbodiimides. The concentration of these reagents is preferably fromabout 0.01M to about 0.2M.

The invention further provides a method of pumping comprising employingan electrode-based pump to move a reagent selected from the groupconsisting of organic amines, such as C1 to C10 hydrocarbons substitutedwith an amino group and carboxylic acids, such as C1 to C10 hydrocarbonssubstituted with a carboxylic acid group. Preferably, the reagent isdissolved in a solvent. In another preferred embodiment, the solvent, inthe absence of the reagent, does not pump using a d.c. poweredelectrode-based pump at a voltage of 2,000 V/mm, more preferably it doesnot pump using a d.c. powered electrode-based pump at a voltage of 4,000V/mm..

Features of other distribution systems described in this application canbe applied to this embodiment, irrespective of under which subheadingthey are described.

B. Hydrologic Liquid Distribution System

One structure in which the invention is usefully employed is ahydrologic liquid distribution system made up of a number of reservoirsand a large number of reaction cells, wherein liquid from any givenreservoir can be systematically directed to all or a substantial subsetof the reactor cells.

Such a liquid distribution system 100 is illustrated in FIGS. 5-10. Thedistribution system is formed of at least three plates, a feedthroughplate 300, a distribution plate 310 and a reaction cell plate 320 (FIG.5). The feedthrough plate 300 is bonded to the distribution plate 310.Most importantly, the feedthrough plate 300 has multiple firstelectrodes 360 and second electrodes 361 that can be manufacturedaccording to the invention. The reaction cell plate 320 is typicallyremovably fitted to the underside of the distribution plate 310, or theunderside of intermediate plate 330 interposed between the distributionplate 310 and the reaction cell plate 320.

FIG. 6 shows the layout of a distribution plate 310. FIG. 7 shows anexpanded view of a portion of a distribution plate 310 that betterillustrates some of the features obscured by the scale of FIG. 6.Typically, the structures indicated in solid lines will be formed in thetop layer of the distribution plate 310, while the structures indicatedwith dotted lines will be formed in the bottom layer of the distributionplate 31 0, except that in FIG. 6 the reaction cells 350 are indicatedby boxes in solid lines even thought these structures are located in alower plane . Where appropriate, vertical channels connect thestructures in the top of the distribution plate 310 with those in thebottom. For convenience, the axis from the top of the illustration tothe bottom is designated the NS axis, while the axis from right to leftis the EW axis.

At the top of FIG. 6 are four first fluid reservoirs 200A, 200B, 200Cand 200D, each having a defined fill level. Each of these first fluidreservoirs 200A, 200B, 200C and 200D has two first reservoir extensions212 extending along substantially all of an EW axis of the distributionplate 310. The ceilings of the first reservoir extensions 212 preferablyare at substantially the same elevation as the first fill level. At fivestaggered locations, A1, B1, C1, D1 and E1, along the EW axis of thefirst reservoir extensions 21 2 there are four first vertical channels214 (not shown) that connect the first reservoir extensions 21 2 withfour first horizontal feeder channel segments 216 that are formed in thebottom layer of the distribution plate 310. At each staggered locationA1, B1, C1, D1 or E1, four adjacent first horizontal feeder channelsegments 216, which are connected to separate first reservoir extensions212, extend along an NS axis to ten positions, A2, B2, C2, D2, E2, F2,G2, H2, I2 and J2. Each position A2, B2, C2, D2, E2, F2, G2, I2 or J2along the course of each such set of four adjacent horizontal feederchannel segments 216 is adjacent to a pair of reaction cells 350 (notshown). At these positions A2, B2, C2, D2, E2, F2, G2, H2, I2, or J2,the four adjacent first horizontal feeder channel segments 216 areseparately connected, via separate second vertical channels 225 (seeFIG. 8), to each of four perpendicular first distribution channels 222formed in the top layer of the distribution plate 310. The ceilings ofthe first distribution channels 222 define a second fill level that istypically substantially the elevation of the first fill level. The filllevel of a distribution channel (e.g., the second fill level) is"substantially" the fill level of the connected reservoir (e.g., thefirst fill level) if they are offset vertically by no more than about10% of the depth of the channel; even if the fill levels are furtheroffset vertically they are still substantially the same if filling thereservoir to its fill level results in filling the connecteddistribution channel and the retention of fluid in the connecteddistribution channel (for instance, retention due to the capillarybarriers described further below with reference to FIG. 8). Thecombination of a first vertical channel 214, connected to a horizontalfeeder channel segment 216, in turn connected to a second verticalchannel 225 makes up a first feeder channel 217 (not identified in theFigures).

If liquids are maintained at a defined first level in a first fluidreservoir 200, then substantially the same level will be maintained inthe first distribution channels 222 connected to that first fluidreservoir 200 via first feeder channels 217. This equalization occursdue to the principle that two connected bodies of liquid will tend toseek the same level and, where the size of the channels allows, due tocapillary flow. Liquids are maintained at a defined level in the firstfluid reservoirs. In the illustrated embodiment, liquid is fed into thefluid reservoir 200 through channels in the feedthrough plate 300 andsuch liquid that is not needed to fill the fluid reservoirs to thedefined level is drained through drains 380. First openings 381 (notshown) are formed in the bottom layer of the feedthrough plate 300 tocreate a liquid connection or sluice between the first fluid reservoirs200 and the drains 380. Liquids are constantly feed into the first fluidreservoirs 200 (as well as the second fluid reservoirs 210 and thirdfluid reservoirs 220) typically by the use of an external pump 15 (notshown), such as the model number 205U multichannel cassette pumpavailable from Watson-Marlow, Inc. Alternatively, a defined level can bemaintained by monitoring the level of liquid in the first fluidreservoirs 200 (or second fluid reservoirs 210 or third fluid reservoirs220) and only activating the pumps feeding liquid to a given fluidreservoir when needed to maintain the defined level.

Each set of four adjacent first distribution channels 222 are adjacentto two buffer channels 218, located to each side of the firstdistribution channels 222 along the EW axis. Liquid can be pumped fromany first distribution channel 222 into the adjacent buffer channel 218by activating the first pump 360 (indicated in FIG. 7 by two filled dotsrepresenting the electrodes of one type of pump) of the firstdistribution channel 222. This pumping creates additional pressure thatmoves the liquid over capillary barrier 370 (see FIG. 8) separating thefirst distribution channel 222 and the buffer channel 218. Between eachfirst distribution channel 222, second distribution channel 224 or thirddistribution channel 226 and the adjacent buffer channel 218 and betweeneach buffer channel 218 and its adjacent third vertical channel 390(described below) there is such a capillary barrier 370 that inhibitsliquid flow when the pumps are not activated. Second openings 362 (seeFIG. 8) are formed in the bottom layer of the feedthrough plate 300 tocreate a liquid connection or sluice between the first distributionchannels 222 and the buffer channels 218. From a buffer channel 218,liquid can be pumped using a second pump 361 (indicated in FIG. 8 by twofilled dots representing the electrodes of one type of pump) to a thirdvertical channel 390 that connects with a reaction cell in the reactioncell plate 320. Third openings 363 (see FIG. 8) in the bottom layer ofthe feedthrough plate 300 or the distribution plate 310 serve to createa liquid connection or sluice between the buffer channels 218 and thirdvertical channels 390.

FIG. 8 illustrates a capillary barrier 370, at which a meniscus 371forms, at the junction between a first distribution channel 222containing liquid 11 and either a buffer channel 218 or a third verticalchannel 390. The meniscus 371 formed at the outlet of first distributionchannel 222 into buffer channel 218 will tend to inhibit seepage fromthe first distribution channel 222, such as the seepage that can resultfrom capillary forces. In some embodiments there are vents (notillustrated) that extend through the feedthrough plate 300 at the topsof buffer channel 218 or third vertical channel 390.

Note that only a small cut-away of NS oriented horizontal feeder channelsegments 216 are shown in FIG. 8. Typically, these channels extendinwardly and outwardly from the illustrated cut-away and connect withadditional first distribution channels 222 situated to distribute liquidto other reaction cells 350.

Along the right side of the distribution plate 310 are ten second fluidreservoirs 210, each having a second reservoir extension 240 extendingalong an EW axis. Second distribution channels 224 form "L"-extensionsoff of second reservoir extensions 240 and are each positioned adjacentto a separate buffer channel 218, such that there are ten seconddistribution channels 224 extending off of each second reservoirextension 240. Each second distribution channel 224 has a pump 360 thatcan move liquid from a second distribution channel 224 to the adjacentbuffer channel 218. Second openings 362 (not shown) in the bottom offeedthrough plate 300 serve to provide a sluice or route of liquidconnection between the second distribution channels 224 and the bufferchannels 218.

Liquid moves from the buffer channels 218 to the reaction cells asdescribed above. Located adjacent to each second reservoir 210 is adrain 380 (not shown) that operates to maintain a defined third filllevel as described above.

As will be described further below in Section D in reference to FIGS.9A-9D, the capillary barriers 370 and sluices created by the secondopenings 362 or third openings 363 act as a combined valve and pump. Thecapillary barriers 370 prevent flow to the reaction cell, which flowwould be favored by capillary forces, until the first pumps 360 orsecond pumps 361 provide the extra pressure needed to overcome thecapillary barriers 370. Narrowing the sluices can increase the capillaryforces favoring flow, thereby reducing the amount of added pressureneeded to overcome the capillary barriers 370. The use of the capillarybarriers 370 allows flow control to be governed by the first pumps 360or second pumps 361, which are typically controlled by controller 10.

Located along the bottom edge of the distribution plate illustrated inFIG. 6 are ten third liquid fluid reservoirs 220. Horizontal feederchannel segments 230 are connected to the third fluid reservoirs 220 andto third distribution channels 226 via fourth vertical channels 227. Thethird distribution channels 226 have first pumps 360 which can moveliquid into adjacent buffer channels 218 via openings 362 (not shown) inthe feedthrough plate 300. Located adjacent to each third fluidreservoir 220 is a drain 380 (not shown) that operates to maintain adefined fourth fill level as described above. Third fluid reservoirs 220and connected third distribution channels 226 operate in just the sameway as first fluid reservoirs 200 and first distribution channels 222.Those of ordinary skill in the art will readily envision alternativegeometries wherein a number of separate third fluid reservoirs 220 caninteract with a given buffer channel 218 via a number of thirddistribution channels 226 positioned adjacent to the buffer channel 218.Located adjacent to each third reservoir 220 is a drain 380 (not shown)that operates to maintain a defined third fill level as described above.

The above discussion describes the distribution system as being formedwith a feedthrough plate 300, distribution plate 310 and reaction cellplate 320. However, it will be clear that additional plates canconveniently be incorporated into the distribution system. For instance,a intermediate plate 330 is, in a preferred embodiment, permanentlybonded underneath the distribution plate 310 and interposed between thedistribution plate 310 and the reaction cell plate 320. The use of theintermediate plate 330 allows for much greater flexibility in the designof the channels the form the distribution system.

C. Controller

The controller 10 will typically be an electronic processor. However, itcan also be a simpler device comprised of timers, switches, solenoidsand the like. The important feature of controller 10 is that it directsthe activity of the first pumps 360 and second pumps 361 and,optionally, the activity of external pumps 171. A circuit of thin filmtransistors (not shown) can be formed on the liquid distribution systemto provide power to the wells via leads and electrodes, and to connectthem with the driving means such as the controller 10, so as to moveliquids through the array. Pins can also be formed substrate which areaddressable by logic circuits that are connected to the controller 10for example.

D. Capillary Barriers

Capillary barriers have been described above with reference to FIG. 8.However, more complex design considerations than were discussed abovecan, in some cases, affect the design of the capillary barrier. In somecases it is desirable to narrow the sluice formed by second opening 362or third opening 363 to increase the impedance to flow (i.e., thefrictional resistance to flow) as appropriate to arrive at anappropriate flow rate when the associated first pump 360 or second pump361 is activated. Such a narrowing is illustrated by comparing thesluice of FIG. 9A with the narrowed sluice of FIG. 9D. The problem thatthis design alteration can create is that narrower channels can increasecapillary forces, thereby limiting the effectiveness of channel breaks.

Thus, in one preferred embodiment, a channel break further includes oneor more upwardly oriented sharp edges 369, as illustrated in FIGS. 9Band 9C. More preferably, a channel break includes two or more upwardlyoriented sharp edges 369. In FIG. 9B, portion 362A of opening 362 is cutmore deeply into first plate 300 to create an open space useful for theoperation of upwardly oriented sharp edges 369.

E. Fabrication of Plates, Channels, Reservoirs and Reaction Cells

The liquid distribution systems of the invention can be constructed asupport material that is, or can be made, resistant to the chemicalssought to be used in the chemical processes to be conducted in thedevice. For all of the above-described embodiments, the preferredsupport material will be one that has shown itself susceptible tomicrofabrication methods that can form channels having cross-sectionaldimensions between about 50 microns and about 250 microns, such asglass, fused silica, quartz, silicon wafer or suitable plastics. Glass,quartz, silicon and plastic support materials are preferably surfacetreated with a suitable treatment reagent such as chloromethylsilane ordichlorodimethylsilane, which minimize the reactive sites on thematerial, including reactive sites that bind to biological moleculessuch as proteins or nucleic acids. As discussed earlier, the expansionvalve liquid distribution system is preferably constructed of a plastic.In embodiments that require relatively densely packed electricaldevices, a non-conducting support material, such as a suitable glass, ispreferred. Corning 211 borosilicate glass, Corning 7740 borosilicateglass, available from Corning Glass Co., Corning, N.Y., are among thepreferred glasses.

The liquid distribution system of the invention is preferablyconstructed from separate plates of materials on which channels,reservoirs and reaction cells are formed, and these plates are laterjoined to form the liquid distribution system. This aspect of theinvention is described in some detail with respect to the hydrologicliquid distribution system. Preferably, the reaction cell plate, e.g.reaction cell plate 320, is the bottom plate and is reversibly joined tothe next plate in the stack. The other plates forming the distributionsystem, which preferably comprise two to three plates are preferablypermanently joined. This joinder can be done, for instance, usingadhesives, such as glass-glass thermal bonding.

One preferred method of permanently joining the plates is to first coatthe plate with a layer of glass glaze generally having a thicknessbetween about 50 microns and about 500 microns, more preferably betweenabout 75 microns and about 125 microns. The above thicknessescontemplate that substantial amounts of channel structure will be formedin the glaze layer. Otherwise, the glaze generally has a thicknessbetween about 1 microns and about 100 microns, more preferably betweenabout 10 microns and about 25 microns. These methods are preferablyapplied to join glass plates. Suitable glazes are available from FerroCorp., Cincinati, Ohio. The glazed plate is treated to create channels,reservoirs, or reaction cells as described below. The glazed plate ispositioned against another plate, which preferably is not glazed, andthe two plates are heated to a temperature of about the softeningtemperature of the glaze or higher, but less than the softeningtemperature for the non-glaze portion of the plates.

Another preferred method of permanently joining glass plates uses afield assisted thermal bonding process. It has now been discovered thatglass-glass sealing using field assist thermal bonding is possibledespite the low conductivity of glass if a field assist bonding materialis interposed between the plates to be bonded.

To the top or bottom surface of one glass plate a layer of a fieldassist bonding material is applied. Preferably, the field assist bondingmaterial layer has a thickness between about 50 nm and about 1,000 nm,more preferably, between about 150 nm and about 500 nm. The field assistbonding material can be a material capable of bonding glass platespursuant to the method described herein. Preferably, the field assistbonding material is silicon or silica. More preferably, the field assistbonding material is silicon.

The field assist bonding material can be applied to a plate, forinstance, by chemical vapor deposition or by a sputtering process wheresurface molecules are emitted from a cathode when the cathode isbombarded with positive ions from a rare gas discharge and the surfacemolecules collide with and bond to a nearby substrate. Pursuant to thepresent invention, silicon layers of between about 150 nm and about 500nm thickness have been bonded to glass plates under conditions that canbe expected to generate an outer surface layer of silicon dioxide, suchas an about 20 Å layer, although the sealing process is believed to beeffective in the absence of this layer. The coated plate is treated, asneeded, to create channels, reservoirs, or reaction cells using themethod described below. Alternatively, the plate was so treated prior tocoating with the field-assist bonding material. The coated plate is thenpositioned against another plate, which preferably is not coated, andplaced in a field assisted bonding device 700 such as that illustratedin FIG. 9. The field assisted bonding device 700 has a heating device710, such as a heating plate. The field assisted bonding device 700further has an electrode 720 and a ground 730 that allows a voltage tobe applied across the first plate 740 and the second plate 750, to whichhas been applied a layer of silicon 760. Generally, the field assistedbonding is conducted under a normal atmosphere.

The plates are brought to a temperature effective when an appropriateelectric field is applied across the plates effective to accelerate thebonding process. While not wishing to be bound by theory, it is believedthat the combination of a cathode applied to the first glass plate 740and the greater exchange-site mobility of ions (such as sodium ions)caused by the elevated temperature causes an ion depletion on the faceof the first glass plate 740 opposite that to which the cathode isapplied. The ion depletion, it is believed, causes a surface charge atthe bottom surface of first glass substrate 740, which correlates withthe creation of a strong localized electrostatic attraction for thesecond substrate 750. It is clear that this process creates strongbonding between the substrates and, it is believed that this is due tothe formation of chemical bonds between the silica of the first glasssubstrate 740 and the silicon coated onto the second glass substrate750. Preferably, the temperature is brought to from about 200° C. toabout 600° C., more preferably from about 300° C. to about 450° C.During the process an voltage typically from about 200 V to about 2,500V, preferably from about 500 V to about 1500 V, is applied across thefirst glass plate 740 and second glass plate 750. The voltage mostsuitably applied varies with the thickness of the glass plates. Thevoltage pulls the first glass plate 740 and second glass plate 750,including the silicon layer 760 applied to one of the plates, intointimate contact. Typically, hermetic sealing is achieved within minutesto about an hour, depending on the planar dimensions of the glassplates. The time required to achieve adequate sealing varies with, amongother things, the smoothness of the plates, the electrical fieldstrength, the temperature, and the dimensions of the plates. Bondingbetween the plates is typically apparent visually, since it isaccompanied by the disappearance of the interface between the plates andthe formation of gray color at the bonded regions that can be seen whenan observer looks through the thinner dimensions of the two plates.

The method described above can be used to bond a glass substrate toanother glass substrate and to a third glass substrate simultaneously.

Those of ordinary skill will recognize that while a hot plate isillustrated as providing the heating for the thermal assisted bonding,other heating devices, including ovens, may be used. It will also berealized that it is desirable to match, when possible, the coefficientsof thermal expansion of the substrates to be bonded.

The reservoirs, reaction cells, horizontal channels and other structuresof the fluid distribution system can be made by the following procedure.A plate, that will for instance make up one of feedthrough plate 300,distribution plate 310, reaction cell plate 320 or intermediate plate330, is coated sequentially on both sides with, first, a thin chromiumlayer of about 500 angstroms thickness and, second, a gold film about2000 angstroms thick in known manner, as by evaporation or chemicalvapor deposition (CVD), to protect the plate from subsequent etchants. Atwo micron layer of a photoresist, such as Dynakem EPA ofHoechst-Celanese Corp., Bridgewater, N.J., is spun on and thephotoresist is exposed, either using a mask or using square orrectangular images, suitably using the MRS 4500 panel stepper availablefrom MRS Technology, Inc., Acton, Mass. After development to formopenings in the resist layer, and baking the resist to remove thesolvent, the gold layer in the openings is etched away using a standardetch of 4 grams of potassium iodide and 1 gram of iodine (I₂) in 25 mlof water. The underlying chromium layer is then separately etched usingan acid chromium etch, such as KTI Chrome Etch of KTI Chemicals, Inc.,Sunnyvale, Calif. The plate is then etched in an ultrasonic bath ofHF--HNO₃ --H₂ O in a ratio by volume of 14:20:66. The use of thisetchant in an ultrasonic bath produces vertical sidewalls for thevarious structures. Etching is continued until the desired etch depth isobtained. Vertical channels are typically formed by laser ablation.

The various horizontal channels of the distribution system embodimentstypically have depths of about 50 microns to about 250 microns,preferably from about 50 microns to about 100 microns, more preferablyfrom about 50 microns to about 80 microns. In an alternative embodiment,the preferred depths are from about 150 microns to about 400 microns.The widths of the horizontal channels and the diameters of the verticalchannels are typically from about 50 microns to about 200 microns,preferably from about 100 microns to about 200 microns, more preferablyfrom about 120 microns to about 150 microns.

F. Fabrication of Electrode-Based Pumps

In many embodiments, the liquid distribution systems of the inventionrequire the formation of numerous electrodes for pumping fluids throughthe liquid distribution system. These electrodes are generallyfabricated in the top glass plate of the liquid distribution system.Typically each pair of electrodes is closely spaced (e.g. 50 to 250microns separation). The electrodes are fabricated with diameters ofpreferably about 25 microns to about 150 microns, more preferably about50 microns to about 75 microns. In preferred embodiments, the liquiddistribution system has 10,000 reaction cell 350 with each reaction cell350 having 6-10 associated electrode-based pumps. Thus, a liquiddistribution system can require about 200,000 to about 300,000electrodes. To produce such structures using mass production techniquesrequires forming the electrodes in a parallel, rather than sequentialfashion. A preferred method of forming the electrodes involves formingthe holes in the plate (e.g., feedthrough plate 300) through which theelectrodes will protrude, filling the holes with a metallic thick filmink (i.e., a so-called "via ink", which is a fluid material thatscinters at a given temperature to form a mass that, upon cooling belowthe scintering temperature, is an electrically conductive solid) andthen firing the plate and ink fill to convert the ink into a goodconductor that also seals the holes against fluid leakage. The methodalso creates portions of the electrodes that protrude through the plateto, on one side, provide the electrodes that will protrude into theliquids in fluid channels and, on the other side, provide contact pointsfor attaching electrical controls.

For example, holes are drilled in 500 micron thick plates ofborosilicate glass using an excimer laser. Holes having diametersbetween 50 and 150 microns are then filled with thick film inks, usingan commercial Injection Via-fill Machine (Pacific Trinetics Model#VF-1000, San Marcos, Calif.). It has been unexpectedly discovered thatonly select formulations of via inks sufficiently function to fill suchhigh aspect ratio holes such that the fired ink adheres to the sides ofthe holes, does not crack during the firing process, and seals the holesagainst fluid flow. One parameter that is important to so formingsealed, conductive conduits through high aspect holes is selecting metalpowder and glass powder components for the via ink that havesufficiently fine dimensions. One parameter that is important to soforming sealed, conductive conduits through high aspect holes isselecting metal powder and glass powder components for the via ink thathave sufficiently fine dimensions. One suitable formulation uses: 12-507Au powder (Technic Inc., Woonsocket, R.I.), 89.3% w/w; F-92 glass (O.Hommel Co., Carnegie, Pa.), 5.7% w/w; 15% w/v ethyl cellulose N-300(N-300, Aqualon, Wilmington, Del.) in Texanol™ (monoisobutarate ester of2,2,4-trimethyl-1,3-pentandiol, Eastman Chemical Products, Kingsport,Tenn.), 2.4% w/w; 15% w/v Elvacite 2045™ (polyisobutyl methacrylate) inTerpineol T-318 (mixed tertiary terpene alcohols, Hercules Inc.,Wilmington, Del.), 2.1% w/w; and Duomeen TDO™ (N-tallow alkyltrimethylenediamine oleates, Akzo Chemicals, Chicago, Ill.), 0.5% w/w.The gold powder from Technic, Inc. has an average particle diameter of0.9 microns. Another suitable formulation uses: Ag Powder Q powder(Metz, South Plainfield, N.J.), 80.8% w/w; F-92 glass (O. Hommel Co.Carnegie, Pa.), 5.2% w/w; VC-1 resin (37% w/w Terpineol T-318, 55.5% w/wbutyl carbitol, 7.5% w/w ethylcellulose N-300, Aqualon, Wilmington,Del.), 3.7% w/w; 15% w/v ethyl cellulose N-300 in Texanol™, 4.0% w/w;15% w/v Elvacite ₂₀₄₅ ™ (polyisobutyl methacrylate) in Terpineol T-318,4.1% w/w; Duomeen TDO™, 0.6% w/w; and Terpineol, 1.6% w/w. Theseformulations were fired at 550° C. to form high aspect ratio conductiveconduits.

When the size of the glass or metal powders increases, good fillingproperties (lack of cracking, good sealing against liquids, goodadherence to sides of hole) can often still be obtained by decreasingthe amount of organic in the via ink.

The devices used to insert via inks into holes in a plate typicallyinclude a metal stencil with openings corresponding to the openings inthe plate. Via ink is applied above the stencil, which rests on theplate, and a bladder device is used to pressurize the ink to force it tofill the holes. After filling, the plate with its via ink-filled holesis removed for further processing, as described below.

Prior to firing, much of the organic component is evaporated away by,for example, placing the ink-filled plate in a oven (e.g. at 100° C.)for one to five minutes. Preferably, the firing is conducted at atemperature from about 450° C. to about 700° C., more preferably fromabout 500° C. to about 550° C. However, the upper end of the appropriatefiring temperature range is primarily dictated by the temperature atwhich the plate being treated would begin to warp. Accordingly, withsome types of plates much higher temperatures could be contemplated.

To assure that there is conductive material that protrudes above andbelow the glass plate after firing, the top and bottom surface of theplate can be coated with a sacrificial layer of thicknesses equaling thelength of the desired protrusions. The sacrificial layers can be appliedbefore or after the holes are formed in the plate. If before, then theholes are formed through both the glass plate and the sacrificiallayers. If after, then (a) corresponding openings through thesacrificial layers can be created by creating a gas pressure differencefrom one side of the plate to the other, which pressure difference blowsclear the sacrificial material covering the holes or (b) such openingsthrough at least the top sacrificial layer are created when the pressureof the ink pushes through the sacrificial layer and into the holes(leaving an innocuous amount of sacrificial layer material in theholes). An appropriate sacrificial layer burns away during the firingprocess. Sacrificial layers can be made coating a plate with, forinstance, 5-25 w/w % mixtures of ethyl cellulose resin (e.g., EthylCellulose N-300, Aqualon, Wilmington, Del.) dissolved in TerpineolT-318™ or Texanol™, or 5-50% w/w mixtures of Elvacite 2045™ in TerpineolT-318™. After firing, the surfaces of the electrode can be enhancedplating metals, such as nickel, silver, gold, platinum, rhodium, etc.The depositions can be performed using standard electrolytic and/orelectroless plating baths and techniques.

Preferably, where a plate that is to contain etched holes will beprocessed to include electrodes, the etching occurs first, followed bycoating with the sacrificial layer and forming the holes.

In an alternate method of manufacture, for each pump, two or more metalwires, for example gold or platinum wires about 1-10 mils in diameter,are inserted into the openings in the channel walls about, e.g., 150microns apart. The wires were sealed into the channels by means of aconventional gold or platinum via fill ink made of finely divided metalparticles in a glass matrix. After applying the via fill ink about thebase of the wire on the outside of the opening, the channel is heated toa temperature above the flow temperature of the via fill ink glass,providing an excellent seal between the wires and the channel. The viaink, which is used to seal the holes, can be substituted with, forinstance, solder or an adhesive.

In an alternate method of manufacture, for each pump, two or more metalwires, for example gold or platinum wires about 1-10 mils in diameter,are inserted into the openings in the channel walls about, e.g., 150microns apart. The wires were sealed into the channels by means of aconventional gold or platinum via fill ink made of finely divided metalparticles in a glass matrix. After applying the via fill ink about thebase of the wire on the outside of the opening, the channel is heated toa temperature above the flow temperature of the via fill ink glass,providing an excellent seal between the wires and the channel.

G. Drivers

An analog driver is can be used to vary the voltage applied to theelectrode-based pump from a DC power source. A transfer function foreach fluid is determined experimentally as that applied voltage thatproduces the desired flow or fluid pressure to the fluid being moved inthe channel. However, an analog driver is required for each pump alongthe channel and is suitably an operational amplifier. Typically,however, a separate analog driver is required for each electrode-basedpump. This is impractical when a large number of channels are to becontrolled.

Thus a digital driver having a pulse of suitable voltage amplitude andthat can provide gating control to the electrodes is preferred for useherein. Control of fluid flow is accomplished by applying pulses ofdifferent pulse widths and different repetition rates to the electrodes.A typical pulse train is shown in FIG. 1 wherein t₁ is the pulse widthand t₂ is the distance between pulses.

FIG. 11 illustrates one configuration for providing control of fluidflow of a plurality of channels simultaneously and independently. Thedata generated for the above variables, as obtained experimentally forvarious fluids and electrodes, is loaded into a controller 10 (notshown), such as a computer. The controller converts the data toinstructions for the digital driver to a first pump 360 or second pump361. The data is transferred to the digital driver and is stored in theshift register 50. Different switching devices 52 attached to eachelectrode pair can be selected, independently of each other, dependingon the state of the latch output. The switching devices are turned onand off by an enabling signal 54 and a latch output signal 55 applied toan AND gate 56. A pulse of a particular width and repetition rate isapplied to the enable signal 54 which determines the length of time theswitch is on or off. Thus the fluid flow in the channel can becontrolled using a signal having constant amplitude but variable pulsewidth and repetition rate. By preselecting the pulse repetition rate, apredetermined applied voltage is selected for each first pump 360 orsecond pump 361 in a channel 10.

An array of the above switching devices 52 can be connected to the shiftregister 50 for controlling the fluid flow of an array of channels, eachswitching device controlling the fluid flow in a different channel. Asingle switching device 52', connected to the shift register 50 througha gate 56', an enable signal 54' and a latch signal 55', is shown forsimplicity in FIG. 5, but a plurality of switching devices will be used,one for each pump in the array of channels.

I. Miscellaneous Features

In the case where the temperature of a particular well is to bemonitored or changed, a means of heating or cooling the well is builtinto the well, as will be further explained below with reference to FIG.20. The first well 36 in this example has deposited on its bottomsurface a thin film 57 of a suitable metal oxide, such as tin oxide orindium tin oxide. The thin film 57 is connected by means of anelectrically conductive metal connection 58 to the end or outer edge ofthe well 36. The tin oxide coating 57 serves as a heater element for thewell 36. The sides of the well 36 have a surface bimetal film 59 andleads 60, suitably made of chromel-alumel alloys, forming a thermocoupleto measure the temperature in the well when a source of current isapplied to the tin oxide coating 57 and to the leads 58. A voltageapplied to the well 36 via electrodes 56 deposited on the backside asshown regulates the temperature in the well. The amount of currentapplied can be regulated by the controller 10 in response to thetemperature measured through the leads 60.

In some applications of the liquid distribution system a significantvapor pressure may develop in reaction cell 350, causing a back pressureinto the distribution plate 310. Thus preformed valves 70 (see FIG. 21A)formed of bimetallic materials as described by Jerman et al,"Understanding Microvalve Technology", Sensors, September 1994 pp 26-36can be situated in third vertical channel 390. These materials have athermal expansion mismatch. When the temperature in the reaction cell350 is low, the ball valve 62 is in its normal position permitting freeflow of fluids into the well 36 (see FIG. 21A). As the temperature inthe well 36 increases, the ball valve 62 moves to a cooler position(FIG. 21B) blocking the third vertical channel 390 to isolate thereaction cell 350, thereby preventing fluids from passing into and outof the first well 36. Alternatively, a conventional check valve having abearing, such as a bearing made of quartz or polytetrafluoroethylenepolymer can be used to isolate the reaction cell 350. Where it isimportant to have the capability to have fluid flow counter to thedirection established by the check valve, the check valve can have aninsulating or magnetic bearing, which can be moved to allow suchcounterflow with externally applied electrostatic-or magnetic fields.

Other features of liquid distribution systems are described in anapplication filed Nov. 9, 1995 entitled, "Liquid Distribution System,"U.S. application Ser. No. 08/556,036, now abandoned, which applicationis a continuation-in-part of U.S. Pat. No. 5,585,069 (application Ser.No. 08/338,703,) titled "A Partitioned Microelectronic and FluidicDevice Array for Clinical Diagnostics and Chemical Synthesis," filedNov. 10, 1994, a continuation-in-part of U.S. Pat. No. 5,632,896(application Ser. No. 08/469,238, ) titled "Apparatus and Methods forControlling Fluid Flow in Microchannels," filed Jun. 6, 1995 and acontinuation-in-part of U.S. application Ser. No. 08/483,331, titled"Method and System for Inhibiting Cross-Contamination in Fluids ofCombinatorial Chemistry Device," filed Jun. 7, 1995. The disclosure ofthis Nov. 9, 1995 application entitled "Liquid Distribution System" andof all the above-recited priority filings named in the Nov. 9, 1995application are incorporated herein by reference in its entirety.

EXAMPLES Example 1--Liquids pumped with a simple electrode-based pump

Using the 1 mm capillary with a two electrode-pump described above inSection B.ii., a number liquids have been tested, including thefollowing solvents:

    ______________________________________    Solvent         Flow direction                               voltage applied    ______________________________________    N-methyl-pyrrolidinone                    +          1470    (NMP)    Dimethyl formamide                    +          390    (DMF)    Dichloromethane (DCM)                    -          686    Methanol (MeOH) -          489    Isopropanol (IPA)                    +    Acetone         +    Acetonitrile    +    ______________________________________

The following solutions in NMP, at 0.1M unless otherwise indicated, havebeen tested:

    ______________________________________    Reagent           Flow direction    ______________________________________    trans-4-(trifluoromethyl)-cinnamic                      -    acid    5-benzimidazolecarboxylic acid                      -    N,N-dicyclohexylcarbodiimide                      +    isobutylamine     +    2-(1H-benzotriazole-1-yl)-                      No flow at 0.1M, flow occurs    1,1,3,3-tetramethyluronium                      lower concentrations (0.01-0.1M)    hexafluorophosphate (HBTU)    ______________________________________

The following solutions in DMF, all at 0.1M excepting piperidine, whichwas 20% v/v, have been tested:

    ______________________________________    Reagent           Flow direction*    ______________________________________    p-carboxybenzenesulfonamide                      -P    4-fluorophenylacetic acid                      -P    4-methoxyphenylacetic acid                      -P    m-trifluoromethytbenzoic acid                      -P    3-(4-methoxyphenyl)propionic                      -    acid    4-bromocinnamic acid                      -P    terephthalic acid -P    isophthalic acid  -P    1,3-phenylenediacetic acid                      -P    1,4-phenytenediacetic acid                      -P    3-(4-carboxyphenyl) propionic                      -P    acid    1,4-phenylenedipropionic acid                      -P    4,4'-oxybis (benzoic acid)                      -P    4,4'-dicarboxybenzophenone                      -P    piperidine        +    1,3-diisopropylcarbodiimide                      +    allylamine        +    butylamine        +    isoamylamine      +    propylamine       +    isobutylamine     +    cyclohexylamine   +    heptylamine       +    benzylamine       +    phenylamine       +P    3-amino-1-propanol                      +P    2-aminoethanol    +    4-(aminomethyl) pyridine                      +P    4-(2-aminoethyl) morpholine                      +P    1-(3-aminopropyl) imidazole                      +    triphenylphosphine                      +    4-(aminopropyl) morpholine                      +    9-fluorenemethanol                      +    p-nitrobenzyl alcohol                      +    p-(methylthio) benzyl alcohol                      -    o-aminobenzyl alcohol                      +    2-methoxybenzyl alcohol                      +    2-(triflouromethyl) benzyl alcohol                      +    2-amino-3-phenyl-1-propanol                      +P    diethylazodicarboxylate                      -P    4-dimethylaminopyridine                      +P    carbazole         +    azobenzene        +    3,4-dihydroxybenzoic acid                      -P    4-methylmorpholine N-oxide                      +    3-cyanobenzoic acid                      No flow    4-nitrophenylacetic acid                      No flow, at 0.1M, flow occurs                      lower concentrations (0.01-0.1M)    2-(1H-benzotriazole-1-yl)-                      No flow, at 0.1M, flow occurs    1,1,3,3-tetramethyluronium                      lower concentrations (0.01-0.1M)    hexafluorophosphate (HBTU)    2,3-dichloro-5,6-dicyano-1,4-benz                      +weak    oquinone    tetrapropylammonium                      No flow    perruthenate    l-oxo-2,2,6,6-tetramethylpiperdini                      No flow    um chloride    5-benzimidazolecarboxylic acid                      N.D..sup.δ    4-(aminomethyl) benzoic acid                      N.D.    4-(aminomethyl) benzoic acid                      N.D.    N,N-diisopropylethylamine                      N.D.    isobutylamine     N.D.    glutathione (SH)  N.D.    ______________________________________     *Those directional indicators ("+" or "-") followed by a "P" indicate tha     flow was achieved using a pulsed voltage program pursuant to FIG. 1, wher     T.sub.1 = 0.1-1 ms and T.sub.2 = 3.0-10 ms.     δ"N.D.", in this table and the tables below, indicates either that     the solute was immiscible with the solvent or that visual inspection     suggested that it had decomposed.

The following solutions in DCM, at 0.1M unless otherwise indicated, havebeen tested:

    ______________________________________    Reagent              Flow direction*    ______________________________________    allylamine           -    butylamine           -    cyclohexylamine      -    1-(3-aminopropyl) imidazole                         -    diethylazodiacarboxylate                         -    TP Palladium         -    isobutylamine        -    isoamylamine         -    propylamine          -    1-(3-aminopropyl)imidazole                         -    p-carboxybenzenesulfonamide                         N.D.    2-(1H-benzotriazole-1-yl)-                         N.D.    1,1,3,3-tetramethyluronium    hexafluorophosphate (HBTU)    ______________________________________     *Those directional indicators ("+" or "-") followed by a "P" indicate tha     flow was achieved using a pulsed voltage program pursuant to FIG. 1, wher     T.sub.1 = 0.1-1 ms and T.sub.2 = 3.0-10 ms.

The following solutions in methanol, all at 0.1M, have been tested:

    ______________________________________    Reagent              Flow direction*    ______________________________________    4-fluorophenylacetic acid                         -    9-fluorenemethanol   -P    p-(methylthio) benzyl alcohol                         -    (R) sec-phenethyl alcohol                         -    3-cyanobenzoic acid  No flow    4-nitrophenylacetic acid                         -weak    allylamine           No flow    2-aminoethanol       No flow    2-(1H-benzotriazole-1-yl)-                         N.D.    1,1,3,3-tetramethyluronium    hexafluorophosphate (HBTU)    isobutylamine        N.D.    isomylamine          N.D.    ______________________________________     *Those directional indicators ("+" or "-") followed by a "P" indicate tha     flow was achieved using a pulsed voltage program pursuant to FIG. 1, wher     T.sub.1 = 0.1-1 ms and T.sub.2 = 3.0-10 ms.

Example 2--An electrode-pump based preferential flow system

A channel system was fabricated on two inch by two inch by 20 mil platesof 211 Corning glass (Corning Glass Co., Corning, N.Y.) to confirm thatliquids can be switched to a desired flow pathway by controlling thevoltages applied to certain electrode-based pumps. As illustrated inFIGS. 11A and 11B, first channel 804 (2,600 μm long by 150 μm wide by100 μm deep), second channel 805 (550 μm long by 100 μm wide by 100 μmdeep), third channel 806 (800 μm long by 275 μm wide by 100 μm deep),fourth channel 807 (200 μm long by 100 μm wide by 100 μm deep), fifthchannel 808 (550 μm long by 100 μm wide by 100 μm deep) and sixthchannel 809 (2,600 μm long by 150 μm wide by 100 μm deep) werefabricated on channel plate 810 (not shown). Also fabricated on thechannel plate 810 were first well 800A, second well 800B and third well800C, which were connected by the channels. An electrode plate 820 wasoverlaid and sealed to the channel plate 810 by field assisted thermalbonding. The electrode plate 820 had openings into first well 800A andsecond well 800B (not illustrated). Third well 800C included a centerdrain 855. The electrode plate 820 further had platinum electrodes,fabricated by inserting 25 μm wires. The electrodes included firstplatinum electrode 801A, second platinum electrode 801B, third platinumelectrode 801C, fourth platinum electrode 802A, fifth platinum electrode802B, third platinum electrode 802C, and the two electrodes comprisinggamma electrode-based pump 803. First platinum electrode 801A, secondplatinum electrode 801B and third platinum electrode 801C make up alphaelectrode-based pump 801, while fourth platinum electrode 802A, fifthplatinum electrode 802B and sixth electrode 802C make up betaelectrode-based pump 802.

FIG. 12A shows methanol flowing from first well 800A to second well800B, while bypassing third well 800C. This is done by applying 160 V toalpha electrode-based pump 801. FIG. 12B shows methanol flowing fromsecond well 800B to third well 800C while bypassing first well 800A.This is done by applying 200 V to beta electrode-based pump 802, 100 Vto gamma electrode-based pump 803 and 120 V to alpha electrode-basedpump 801, where the polarity at beta and gamma electrode-based pumps 802and 803 favored flow into the third well 800C, and the polarity at alphaelectrode-based pump 801 favored flow away from first well 800A.

Example 3--Electrode-based Dumping past capillary barriers

FIG. 13 shows a prototype liquid distribution system fabricated pursuantto the hydrologic liquid distribution system. The distribution systemwas constructed from three plates of Corning 7740 borosilicate glass,Corning Glass, Inc., Corning, N.Y. which plates became top plate 910,intermediate plate 920 and bottom plate 930. The top of intermediateplate 920 was coated with silicon as described above. In top plate 910were formed, by laser drilling, first hole 901A, second hole 901B, thirdhole 902A, fourth hole 902B, fifth hole 903A, sixth hole 903B, seventhhole 904A and eighth hole 904B, which holes each had a diameter of 75 μmFirst and second holes 901A and 901B were used to form first modelelectrode-based pump 961. Third and fourth holes 902A and 902B were usedto form second prototype electrode-based pump 962. Fifth and sixth holes903A and 903B were used to form third prototype electrode-based pump963. Seventh and eighth holes 904A and 904B were used to form fourthmodel prototype electrode-based pump 964. The electrodes in each offirst through fourth prototype electrode-based pumps, 961-964, wereseparated by 200 μm. By etching, alpha opening 905, beta opening 906 andgamma opening 907 were formed on the underside of top plate 910. Bylaser drilling, ninth hole 908 and tenth hole 909, each with a diameterof 150 μm, were formed through upper plate 910.

In intermediate plate 920 were formed first prototype channel 911 (madeup of segments 911A-911D) and second prototype channel 912 (made up ofsegments 912A-912D). First and second prototype channels 911 and 912having a depth of 80 μm and a width of 150 μm. The entries into thesetwo prototype channels 911 and 912 are provided by ninth hole 908 andtenth hole 909, respectively. First reaction cell access hole 913 andsecond reaction cell access hole 914, each with a diameter of 150 μm,were laser drilled through the intermediate plate 920. In the undersideof intermediate plate 920, a delta opening 915 was formed, which deltaopening 915 connects the reaction cell 950 to first and second prototypedrain holes 921 and 922.

In the bottom plate 930, the reaction cell 950 was formed by etching.First prototype drain hole 921 and second prototype drain hole 922 werelaser drilled through bottom plate 920. The top plate 910 andintermediate plate 920 were bonded together by field assisted thermalbonding.

When methanol was introduced into first prototype channel 911, theliquid was stopped from flowing into reaction cell access hole 913 bythe capillary barrier formed by the structure at alpha opening 905.Correspondingly, the capillary barrier formed by the structure at betaopening 906 prevented methanol flow into the reaction cell access hole914. Flow into the reaction cell access holes 913 or 914, by eitherroute, could be initiated by activating the appropriate pumps. Forinstance, to pump methanol through first prototype channel 911, firstprototype electrode-based pump 901 and second prototype electrode-basedpump 902 were biased by applying 200 V. Flow through the prototypechannel 911 was observed.

In the claims:
 1. An electrode-based pump comprising, in a substrate,(a)a first electrode and a second electrode intersecting with a channel ofcapillary dimensions, wherein the first and second electrodes have adiameter from about 25 microns to about 100 microns and are spaced fromabout 100 microns to about 2,500 microns apart, and wherein the pumpoperates to move liquid in the channel as a result of a potentialapplied between the first and second electrodes; and (b) a voltagedriver for applying voltage across the electrodes,wherein the pump inone mode of operation is adapted to pump fluid by having the driverapply voltage to just said first and second electrodes.
 2. Theelectrode-based pump of claim 1, wherein the first and second electrodesare from about 150 microns to about 1000 microns apart.
 3. Theelectrode-based pump of claim 1, wherein the first and second electrodesare from about 150 microns to about 250 microns apart.
 4. Theelectrode-based pump of claim 1, wherein the electrodes are formed offused via ink.
 5. The electrode-based pump of claim 4, wherein thechannel intersecting portion of the fused via ink is plated with metal.6. The electrode-based pump of claim 5, wherein the plated metal isnickel, silver, gold, platinum or rhodium.
 7. The electrode-based pumpof claim 4, wherein the fused via ink extends through a via in thesubstrate that intersects with the channels, and wherein the fused viaink seals the vias against liquid leakage from the channel.
 8. Theelectrode-based pump of claim 1, wherein the voltage driver is forapplying voltage pulses across the electrodes.
 9. The electrode-basedpump of claim 8, wherein the driver is a digital driver.
 10. Anelectrode-based pump comprising:a substrate in which a channel ofcapillary dimensions is formed; electrical conduits through thesubstrate intersecting with the channel at points separated by fromabout 100 microns to about 2,500 microns formed of fused via ink,wherein the fused via ink seals the conduits against liquid leakage fromthe channel; and metal plating on ends of the fused via ink intersectingthe channel, wherein the metal platings and associated conduits defineelectrodes.
 11. The electrode-based pump of claim 10, wherein the firstand second electrodes are from about 150 microns to about 1000 micronsapart.
 12. The electrode-based pump of claim 10, wherein the first andsecond electrodes are from about 150 microns to about 250 microns apart.